The present disclosure relates to soldering, in particular to an apparatus and method for soldering chips to a substrate.
Simple flexible systems, such as logic functions with transistors or optoelectronic devices, can in principle be fully printed on a substrate (e.g. foil or rigid board). However, for more complex systems, there is a need to develop hybrid systems in which printed circuitry is combined with silicon-based integrated circuits or surface mounted device (SMD) components, referred herein as chip components or simply “chips”. To functionalize the device, multiple chip components, often having different dimensions, may need to be interconnected to the circuit tracks on the substrate, e.g. printed or etched copper circuits. This can be realized for example using oven reflow soldering, conducting adhesive bonding or face-up chip integration. However, these processes are considered time consuming and/or incompatible with low-cost polyester foils having low decomposition temperatures. Especially for soldering process, commonly used polymer substrates tend to degrade and deform under thermal load above 150° C.
For example, reflow soldering can typically be used for interconnecting thick chips on a rigid substrate board such as FR4 or ceramics. However, reflow soldering, is poorly compatible with low-cost flexible foils and roll-to-roll (R2R) processing because it requires maintaining the whole board above the liquidus temperature of the solder which is generally above 200° C. for long holding time. This typically results in a time-consuming process using large in-line ovens, often having multiple loops. The long holding time may also cause deformation or degradation of the flexible foil itself or degradation of its organic surface coatings or adhesives. It is considered impossible to oven reflow conventional solder on low-cost polyester foils, such as polyethylene terephthalate (PET), by using industry standard lead-free alloys because PET has a maximum processing temperature of around 120° C.-150° C., which is much lower than the liquidus temperature of these solders (>200° C.).
As an alternative, for example, infrared (IR) heating can be used with comparable soldering time. For example, infrared laser spots can be used to heat each solder connection sequentially. However, in laser spot soldering, the small spot area may require precise positioning of the spot for each component. Furthermore, applying this technology in a R2R process is challenging as the laser spot needs to align to multiple chips on a moving substrate. Furthermore, the process can be time consuming. Accordingly, some of these methods may still be based on stop and go.
As another alternative, large area illumination by a high-energy light pulse of a flash lamp can be used. For example, an article in Electronic Materials Letters, Vol. 10, No. 6 (2014), pp. 1175-1183 by Van den Ende et al. describes Large area photonic flash soldering of thin chips on flex foils for flexible electronic systems. Advantageously, when the timescale of the heating pulse is short enough to avoid diffusive heating of the flexible polymer substrate, components can be soldered at temperatures higher than the maximum processing temperature of the foils. However, if the absorption of light by the (foil) substrate and/or components differs, this can lead to selective heating. Furthermore, electronic devices, generally consist of multiple chip components. This may lead to further differences in heating behaviour for the different components which makes the temperature and solder process difficult to control.
Accordingly, there remains a need for improvement in the soldering of chips to a substrate, e.g. faster, more reliable, compatible with flexible foil substrates, roll-to-roll processing, and different chips and/or substrates.